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United States Patent |
6,143,581
|
Johnson
,   et al.
|
November 7, 2000
|
Asymmetric transfer molding method and an asymmetric encapsulation made
therefrom
Abstract
A method of encapsulating an article having first and second surfaces,
includes positioning a first molding section in a sealing relationship
with the first surface of the article and positioning a second molding
section adjacent the second surface of the article. The first molding
section is filled first thereby forcing the second surface of the article
into a sealing engagement with the second molding section. The second
molding section is then filled.
Inventors:
|
Johnson; Mark S. (Boise, ID);
Bolken; Todd O. (Meridian, ID)
|
Assignee:
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Micron Technology, Inc. (Boise, ID)
|
Appl. No.:
|
255554 |
Filed:
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February 22, 1999 |
Current U.S. Class: |
438/26; 257/E21.502; 257/E21.504; 257/E23.119; 257/E23.125; 438/22; 438/124; 438/127 |
Intern'l Class: |
H01L 021/00 |
Field of Search: |
438/26,22,124,127
|
References Cited
U.S. Patent Documents
4744637 | May., 1988 | Sekimura et al.
| |
5070041 | Dec., 1991 | Katayama et al.
| |
5270573 | Dec., 1993 | Takayanagi et al.
| |
5766987 | Jun., 1998 | Mitchell et al.
| |
5874324 | Feb., 1999 | Osada | 438/124.
|
Other References
John H. Lau(Editor), Chip on Board Technologies for Multichie Modules; 1994
pp. 2, 3, 24-27, 30-33, 330-337; Van Nostrand Reinhold, an International
Thomson Publishing Company.
|
Primary Examiner: Picardat; Kevin M.
Assistant Examiner: Collins; D. M.
Attorney, Agent or Firm: Thorp Reed & Armstrong
Claims
What is claimed is:
1. A method of encapsulating an article having first and second surfaces,
comprising:
positioning a first molding section in a sealing relationship with the
first surface of the article;
positioning a second molding section adjacent the second surface of the
article;
filling the first molding section thereby forcing the second surface of the
article into a sealing engagement with the second molding section; and
filling the second molding section.
2. The method of claim 1 wherein said step of filling said first and second
molding sections includes the step of filling said first and second
molding sections with an encapsulating material chosen from a class
consisting of epoxies, including thermo-set resins, silicones, sycar,
polyimides, and polyurethanes.
3. The method of claim 2 including the step of curing said encapsulating
material.
4. A method of encapsulating an electrical device positioned on a flexible
substrate, comprising:
positioning a portion of the flexible substrate carrying the electrical
device within a molding cavity defined by first and second molding
sections, said substrate forming a seal with the first molding section;
forming a seal between the substrate and the second molding section by
filling the first molding section; and
filling the second molding section.
5. The method of claim 4 wherein said step of filling said first and second
molding sections includes the step of filling said first and second
molding sections with a thermo-set resin.
6. The method of claim 4 including the step of curing said thermo-set
resin.
7. A method, comprising:
mounting an electrical device on a first surface of a flexible substrate
such that leads from the electrical device extend to a second surface of
the flexible substrate;
connecting the leads to traces formed in the second surface of the flexible
substrate;
positioning a portion of the flexible substrate carrying the electrical
device within a molding cavity defined by first and second molding
sections, said substrate forming a seal with the first molding section;
forming a seal between the substrate and the second molding section by
filling the first molding section; and
filling the second molding section.
8. The method of claim 7 wherein said step of positioning a portion of the
flexible substrate carrying the electrical device within a molding cavity
results in the electrical device being located completely within the first
molding section.
9. The method of claim 8 wherein said step of positioning a portion of the
flexible substrate carrying the electrical device within a molding cavity
results in the leads and connections to traces being located completely
within the second molding section.
10. The method of claim 7 wherein said step of filling said first and
second molding sections includes the step of filling said first and second
molding sections with a thermo-set resin to form a molded article.
11. The method of claim 10 including the step of curing said thermo-set
resin.
12. The method of claim 7 including the step of removing said molded
article from the molding cavity.
13. The method of claim 7 including the step of separating said portion of
the flexible substrate carrying the electrical device from the remaining
portion of the substrate.
14. The method of claim 7 wherein said step of positioning said portion of
the flexible substrate carrying the electrical device within said molding
cavity results in the creation of a path between the portion of the cavity
defined by said first molding section and the portion of the cavity
defined by said second molding section.
15. A method of dynamic encapsulation, comprising:
filling a first molding cavity positioned in a sealing relationship with an
article to be encapsulated;
dynamically creating a sealing relationship between the article and a
second molding cavity; and
filling the second molding cavity.
16. The method of claim 15 wherein said step of dynamically creating a
sealing relationship includes the step of allowing the article to flex
into a sealing engagement with the second molding cavity.
17. A method, comprising:
mounting an electrical device to a first surface of a flexible substrate
such that leads from the electrical device extend to a second surface of
the flexible substrate;
connecting the leads to traces formed in the second surface of the flexible
substrate;
positioning a portion of the flexible substrate carrying the electrical
device within a molding cavity defined by first and second molding
sections, said substrate forming a seal with the first molding section;
filling the first molding cavity to dynamically create a sealing
relationship between the flexible substrate and the second molding cavity;
and
filling the second molding cavity.
18. The method of claim 17 wherein said step of positioning a portion of
the flexible substrate carrying the electrical device within a molding
cavity results in the electrical device being located completely within
the first molding section.
19. The method of claim 18 wherein said step of positioning a portion of
the flexible substrate carrying the electrical device within a molding
cavity results in the leads and connections to traces being located
completely within the second molding section.
20. A method of transfer molding, comprising:
positioning a first portion of encapsulated material on a first surface by
transfer molding; and
positioning a second portion of encapsulated material, of smaller surface
area than said first portion, on a second surface by transfer molding.
21. A method of asymmetric transfer molding, comprising:
positioning an article to be encapsulated within a molding cavity defined
by first and second mold sections, said article forming a seal with said
first mold section;
filling said first mold section by transfer molding, said first mold
section defining a first surface area; and
filling said second mold section by transfer molding, said second mold
section defining a second surface area, said second surface area being
smaller than said first surface area of said first mold section.
22. The method of claim 21 wherein said steps of filling said first and
second mold sections each include the step of filling said first and
second mold sections with a thermo-set resin.
23. The method of claim 21 wherein said second surface area of said second
mold section is approximately three times smaller than said first surface
area of said first mold section.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of encapsulating an article and, more
specifically, to transfer molding an asymmetric encapsulation for an
electronic device, and an asymmetric encapsulated electronic device.
2. Description of the Background
It is well known that electronic devices are sensitive and thus require
protection from physical and environmental conditions which may degrade or
completely ruin them. Therefore, it is well known in the art to protect
electronic devices from these conditions by sealing them with a protective
encapsulation material. This "packaging" of the electronic devices
protects them from the conditions which may degrade them and allows the
devices to be transported and handled, and thus allows them to be easily
configured with other components. Several encapsulation methods are known
in the art such as under-fill encapsulation (for flip chip applications)
and glob topping.
One prior-art method for encapsulating devices is the "transfer-molding"
method. Transfer molding is a process through which an encapsulating
material, such as a thermosetting material, is caused to flow into a
cavity formed by components of a mold. The thermosetting material enters
into the cavity and flows over the electronic device that is located
within the cavity and is then "cured" so that the resin hardens into a
non-flowable state. Traditionally it has been important to control the
flow of the material into the cavity for a number of reasons, including:
to provide void-free fill over the electrical device, to control the flow
of the material so as to not contaminate unwanted areas with the
encapsulating material, and to control the flow of the material so as to
not cause any wire displacement or other damage to the assembly.
Also, it is well known in the art to use mechanical clamping mechanisms as
sealing devices in conjunction with the molding cavity to attempt to
contain the thermosetting resin within the cavity prior to curing.
However, because mechanical clamps must be applied in a symmetrical
fashion, i.e. equal and opposite clamping forces, the configuration of the
molds is limited to symmetrical designs.
Therefore there is a need for an improved encapsulation method for transfer
molding electronic devices which provides a leak proof sealing mechanism
for asymmetric designs while using current equipment and known materials
and techniques.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a method of
encapsulating an article having first and second surfaces includes the
following steps: positioning a first molding section in a sealing
relationship with the first surface of the article; positioning a second
molding section adjacent the second surface of the article; filling the
first molding section thereby forcing the second surface of the article
into a sealing engagement with the second molding section; and filling the
second molding section. Additionally, the step of filling the first and
second molding sections may include the step of filling the first and
second molding sections with an encapsulating material chosen from a class
consisting of epoxies (including thermo-set resins), silicones, sycar,
polyimides, and polyurethanes. Also, depending on the type of
encapsulating material that is chosen, an optional step of curing the
encapsulating material may be required.
Another aspect of the present invention is to provide a method which
includes the following steps: mounting an electrical device to a first
surface of a flexible substrate such that leads from the electrical device
extend to a second surface of the flexible substrate; connecting the leads
to traces formed in the second surface of the flexible substrate;
positioning a portion of the flexible substrate carrying the electrical
device within a molding cavity defined by first and second molding
sections, the substrate forming a seal with the first molding section;
forming a seal between the substrate and the second molding section by
filling the first molding section; and filling the second molding section.
Another aspect of the present invention is to provide a method of transfer
molding, including the steps of: positioning a first portion of
encapsulated material on a first surface by transfer molding; and
positioning a second portion of encapsulated material, of smaller surface
area than the first portion, on a second surface by transfer molding.
Another aspect of the present invention is to provide a mold, which
includes: a first molding section and a second molding section. The first
and second molding sections form a cavity for receiving an article to be
encapsulated. The surface area of a first side of an article exposed to
the cavity is greater than the surface area of a second side of the
article exposed to the cavity. At least one gate for injecting
encapsulating material into the cavity is provided.
Another aspect of the present invention is to provide a non-symmetrical,
transfer molded, encapsulated article.
The present invention provides several advantages over prior art
techniques. Yields are increased in comparison to parts which are glob
topped. The method of the present invention may be carried out using
existing processes and materials having well known and understood
properties. Additionally, existing molding equipment may be used. Assembly
throughput is increased. Those advantages and benefits, and others, will
be apparent to those of ordinary skill in the art from the Description of
a Preferred Embodiment, herein below.
BRIEF DESCRIPTION OF THE DRAWINGS
For the present invention to be readily understood and practiced, the
invention will now be described, for purposes of illustration and not
limitation, in conjunction with the following figures wherein:
FIG. 1 illustrates a substrate which may be used in conjunction with the
method of the present invention;
FIG. 2 illustrates a portion of the substrate of FIG. 1 carrying an
electrical device to be encapsulated;
FIG. 3 illustrates the opposite side of the substrate illustrated in FIG.
2;
FIG. 4 illustrates first and second mold sections which define a mold
cavity;
FIG. 5 illustrates the portion of the substrate shown in FIGS. 2 and 3
positioned in the mold cavity;
FIG. 6 illustrates the sequence of filling the mold cavity;
FIGS. 7 and 8 are perspective and plan views, respectively, of a first side
of the portion of the substrate illustrated in FIGS. 2 and 3, after
encapsulation;
FIGS. 9 and 10 are perspective and plan views, respectively, of a second
side of the portion of the substrate illustrated in FIGS. 2 and 3, after
encapsulation;
FIGS. 11 and 12 are views taken along the lines XI and XII, respectively,
in FIG. 10;
FIGS. 13A and 13B illustrate an asymmetric encapsulated device; and
FIG. 14 illustrates another gate arrangement which may be used with the
method of the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
FIG. 1 illustrates a substrate 10 which may be used in conjunction with the
method of the present invention. The description of the method of the
present invention in connection with the substrate 10 is for purposes of
illustration only, and not limitation. It is anticipated that the method
and molds of the present invention may be used to encapsulate a large
variety of articles, both electrical and non-electrical. In FIG. 1, the
substrate is a known flexible substrate suitable for receiving electrical
devices 12 in openings 14. The electrical device may be any type of
device, although a memory device is shown in FIG. 1.
The electrical device 12 may be connected to a first side 16 of the
substrate 10 by any known means, e.g. lamination, adhesion, etc. The
device 12 may be of a type in which electrical connections extend from the
center of the device. The device 12 is positioned such that the electrical
connections 18 of the device 12 extend through opening 14 to a second side
20 of substrate 10, seen in FIG. 3. In the remaining pre-singulation
figures, only portion 22 of substrate 10 is illustrated for purposes of
convenience. The reader will recognize that the description of the method
and molds hereinafter with respect to portion 22 is actually carried out
"x" times, e.g., 8, 10, 12, etc. depending upon the size and capacity of
the encapsulating equipment.
In FIG. 2 it is seen that the device 12 is positioned such that opening 14
is not completely blocked leaving a small opening referred to as a
secondary gate 24. If the device 12 completely blocks opening 14, then an
alternate means of providing encapsulating material must be provided as
described below.
In FIG. 3, the second side 20 of the portion 22 of the substrate 10 is
illustrated. Second side 20 has traces 26 formed therein. The traces 26
may be formed using a solder masking step as is known in the art. Solder
balls 28 may be embedded in portion 22 to provide a termination/connection
point for each of the traces 26. After the device 12 is connected to the
portion 22 and the electrical connections 18 are extending through opening
14, each electrical connection 18 is connected to one of the traces using
any known connection technique and machinery.
A mold 30 which may be used in conjunction with the method of the present
invention is illustrated in FIG. 4. In FIG. 4, a first mold section 32 and
a second mold section 34 cooperate to define a mold cavity 36.
FIG. 5 illustrates the portion 22 positioned within cavity 36. The portion
22 is positioned such that device 12 is entirely within the portion of the
cavity 36 formed by the first mold section 32, although, for other devices
that may not be the case. Similarly, opening 14, and the connection of the
electrical connections 18 to traces 26 is positioned entirely within the
portion of the cavity 36 formed by the second mold section 34.
The asymmetry of the mold sections 32, 34 can be clearly seen in FIGS. 4
and 5. In this illustrated embodiment, the surface area of the mold
section 32 is approximately three times greater than a surface area of the
mold section 34. Generally the surface area (A16) of the first side 16 of
portion 22 facing mold cavity 36 must be greater than the surface area
(A20) of the second side 20 of portion 22 facing mold cavity 36. That is
A16>A20.
Another by-product of the asymmetry is that first mold portion 32 may be
sealed against the first surface 16 of portion 22 by applying clamping
pressure in the four areas marked 38. No such seal can be formed at this
time between second mold portion 34 and the second surface 20 of portion
22 because no clamping pressure can be exerted in opposition to the two
areas marked 40. Not shown in FIGS. 4 and 5 is a runner and primary gate,
which is the mechanism for injecting the encapsulating material into the
portion of the mold cavity formed by the first mold section 32.
Turning now to FIG. 6, the sequential flow of encapsulating material into
the mold cavity 34 is shown. Encapsulating material flows under pressure
through runner 42 into the portion of the mold cavity formed by first mold
section 32. The pressure in runner 42 is designated P1 while the pressure
in the portion of the mold cavity formed by first mold section 32 is
designated P2. The encapsulating material passes through secondary gate 24
at a pressure of P3 and into the portion of the mold cavity formed by
second mold section 34 at a pressure of P4. The pressure is controlled
such that P1>P2>P3>P4.
In one embodiment, the encapsulation material is a thermo-set epoxy resin
mixture and may be loaded under a pressure in the range of 500-2000 psi.
Fill times when the encapsulation material is a thermoset epoxy resin
mixture are on the order of 3-10 seconds. The injection pressure and fill
times are dependent upon the specific encapsulating material that is used.
As the encapsulation material fills the portion of the mold cavity formed
by the first mold section 32, the portion 22 of the substrate 10 bends or
flexes under the pressure exerted by the encapsulation material. The
bending brings the second surface 20 into a sealing engagement with the
second molding section 34. Thus, a seal is dynamically formed as a portion
of the mold cavity formed by the first mold section 32 is filled. In
low-pressure applications, it is anticipated that a force may be exerted
to cause the bending to take place.
The encapsulation material may be chosen from a class consisting of epoxies
(including thermo-set resins), silicones, sycar, polyimides, and
polyurethanes. These encapsulation materials are suitable for use when
encapsulating electronic components because they have low moisture
permeability, high mobile ions barriers, good UV-VIS and alpha particle
protection, favorable mechanical, electrical and physical properties, as
well as a low dielectric constant to reduce the device propagation delay
and high thermal conductivity to dissipate heat generated by the devices.
The proper choice of encapsulation material can enhance reliability of the
device and improve its mechanical and physical properties. An optional
curing step may hereinafter be required, depending upon the choice of
encapsulation material, followed by removal of the mold sections 32, 34.
After the mold sections 32, 34 are removed, the encapsulated electronic
device appears as shown in FIGS. 7, 8, 9, and 10. FIGS. 7 and 8 show the
first side 16 of portion 22 after removal of mold section 32. A first
portion of encapsulated material 33 is formed as a result of the
encapsulation material being loaded into mold section 32. FIGS. 9 and 10
show the second side 20 of portion 22 after removal of mold section 34. A
second portion of encapsulated material 35 is formed as a result of the
encapsulation material being loaded into mold section 34.
FIGS. 11 and 12 illustrate views taken along the lines XI and XII,
respectively, in FIG. 10. FIG. 12 provides a view of runner 42 and primary
gate 44, which is the mechanism for injecting the encapsulation material
into the portion of the mold cavity formed by the first mold section 32.
FIGS. 13A and 13B show the final product of the present invention after
singulation, i.e. after portions of the substrate 22 are separated and the
runner 42 is removed.
FIG. 14 shows another gate arrangement which may be used with the method of
the present invention. The electrical device to be encapsulated is a
memory device identical to the one as shown in previous FIGS. 1 through
13B, although the electrical device may be any type of device. The
electrical device 12 is positioned such that opening 14 (see FIG. 1) is
completely blocked. Thus encapsulating material injected into first mold
section 32 from runner 42 and primary gate 44 is contained within first
mold section 32. That is, there is no connecting gate between first mold
section 32 and second mold section 34 through which encapsulation material
may flow. Encapsulating material may be injected directly into mold
section 34 through a gate 46 and a runner, not shown. The encapsulating
material injected into mold section 34 is contained within mold section
34.
While the present invention has been described in conjunction with
preferred embodiments thereof, those of ordinary skill will recognize that
many modifications and variations thereof are possible. For example, a
limitless number of asymmetric mold designs are possible. It is
anticipated that the method of the present invention may be carried out
using a variety of encapsulating materials and commercially available
injection molding machines. Also, numerous other gate configurations are
possible. The foregoing description and the following claims are intended
to cover all such modifications and variations.
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